The clinical use of methylphenidate (MPH, Ritalin) for treatment of attention-deficit/hyperactivity disorder is widespread, and there is a growing problem of MPH abuse, especially in college-age adults. College students use MPH non-medically, mainly to enhance performance, stay up late to study or to get high. In addition, adults of all ages are using high doses of MPH off-label for energy and cognitive enhancement. The abused dosages of MPH are 2-10 times those recommended for clinical use, however, little is known about the neurobiological effects of chronic exposure to these MPH doses. Our laboratory has recently discovered an unexpected consequence of chronic high-dose MPH treatment in mice. We have found that the behavioral and neurochemical responses to fluoxetine are qualitatively transformed. Fluoxetine is a serotonin (5-HT) transporter inhibitor which normally reduces extracellular nucleus accumbens (NAc) dopamine (DA) levels in control animals and fails to act as a reinforcer. Remarkably, however, we found that following chronic MPH treatment, fluoxetine takes on the characteristics of a psychostimulant drug, exhibiting rewarding effects as well as DA-elevating effects. Given that serotonergic drugs often suppress the reinforcing effects of DA agonists, these and other data suggest the possibility of a fundamental alteration in 5-HT-DA interactions whereby activation of the 5-HT system leads to elevated DA levels in limbic brain areas and activation of reward-related processes. We hypothesize that chronically elevated DA levels causes 5-HT1B receptors in the VTA to become supersensitive and their activation stimulates DA release into the NAc and other DA terminal regions. We hypothesize that this change could specifically increase the reinforcing effects of drugs with strong 5-HT activity such as MDMA, potentially leading to enhanced risk of polydrug abuse in people taking MPH. To explore the impact of chronic MPH treatment in mice on specific interactions between the 5-HT and DA systems, and to explore the neurochemical and behavioral consequences of MPH self-administration in rats, we propose to examine 1) 5-HT alterations in response to i.p. MPH treatment in mice, 2) Sites of 5-HT action, using dual probe microdialysis in mice 3) MPH self-administration in rats 4) 5-HT alterations in response to MPH self-administration.